1. Field of the Invention
The present invention relates to a six-axis force sensor mounted on the fingertip of a human for analyzing a grasping operation made by the human and detecting a contact force during the grasping operation so that a robot can conduct the same grasping operation in accordance with the analyzed and detected data. This sensor is also applicable to a force display device that gives a human a sensation of actual existence of virtual data stored in a computer.
2. Description of the Prior Art
As conventional six-axis force sensors, there have been developed the one mounted on the wrist of a robot (e.g. six-axis force sensor IFS series by NITTA Co.) and the one mounted on the fingertip of a robot hand (e.g. NANO sensors by BL Autotech Ltd.). As conventional tactile sensors mounted on a hand of a human being to detect distribution of pressures applied onto the fingers and palm, a glove-shaped tactile sensor for pressure distribution using pressure-sensible conductive rubber and a conductive ink sheet (e.g. the glove scan system by NITTA Co.) has been developed, for example.
In order to skillfully manipulate a given object with the fingertips of a robot hand, it is at first necessary to analyze a grasping operation conducted with a hand of a human being. In order to analyze such human manipulation, it is necessary to detect the finger motion and contact force acting on the fingertip.
The glove scan system developed by NITTA Co. has been cited above as a sensor for detecting a pressure distribution onto a hand of a human being. Another example is a sensor glove having a distributed tactile sensor sewed onto a glove [Shimizu et al., xe2x80x9cDevelopment of Sensor Glove MK III for measuring grasping pressure distribution,xe2x80x9d The 14th Science Lecture Meeting of the Robotics Society of Japan, 1996].
These sensor gloves can detect distribution of pressures applied onto the fingers and palm. However, the detected force components are those of the force exerted only in the direction perpendicular to the surface of the sensor. In other words, forces exerted in the direction horizontal to the sensor surface, including a shearing force and a frictional force, and a contact moment on the sensor surface cannot be detected by the sensor gloves.
On the other hand, it has been known that the grasping force used when a human being grasps a given object is a critical point to induce a slip between the given object and the fingers [Yamada, xe2x80x9cDetection of Slip and Static Friction Coefficient,xe2x80x9dJournal of the Robotics Society of Japan, Vol. 11, No. 7, 1993]. This indicates that a human being pays his attention to not only the perpendicular force, but also the horizontal force relative to the finger surface when grasping a given object.
It has also pointed out that a human being utilizes a frictional force (the force in the direction horizontal to the finger surface) and a contact moment on the finger surfaces when manipulating a given object. In order to use data detected from demonstration manipulation by a human being directly for the control of a robot hand, it is desirable that data of a sensor identical with the sensor mounted on the robot hand be analyzed.
It is reported that it is important to use a six-axis force sensor as a sensor to be mounted on the fingertip of a robot hand [Nagata et al., xe2x80x9cDevelopment of a Fingertip-type 6D Force Sensor and Error Evaluation of Contact Point Sensing,xe2x80x9d Journal of the Robotisc Society of Japan, Vol. 11, No. 7, 1993]. From this point of view, it has been desired to use, as a sensor mounted on a hand of a human being for detecting the contact force, a six-axis force sensor that can detect forces and moments in the three axial directions
There have heretofore been developed six-axis force sensors for robots. Of these sensors, NANO sensors produced by BL Autotech Ltd. have the smallest size that is 18 mm in diameter and 32.8 mm in length. This size is too large to mount such a NANO sensor on the fingertip of a human being.
The present invention has been accomplished in view of the above. An object of the present invention is to provide a six-axis force sensor capable of being mounted on the fingertip of a human being.
To attain the above object, the present invention provides a fingertip-mounted six-axis force sensor comprising a fingerstall into which a fingertip of a human being is inserted, an elastic component connected and fixed to the fingerstall, easy to distort in response to force components (that comprise fingertip contact forces and contact moments throughout the description) and equipped with distortion detecting means, and a finger cover equipped with a fitting block and connected and fixed to the elastic component via the fitting block, wherein distortion of the elastic component produced in response to the contact force and moment exerted onto an object by the human fingertip is detected by the distortion detecting means and converted into an electrical signal that is taken out and calculation-processed to detect the fingertip contact forces.
The elastic component may comprise a base connected to the fingerstall, a peripheral ring fixed to the fitting block, four beams that connect the base and the peripheral ring, and distortion detecting means, such as strain gauges or optical sensor units, attached to the surfaces of each beam. When a force is exerted onto the elastic component in the direction of the x-axis or y-axis, the two beams disposed perpendicular to the force-exerted direction distort. When a force is exerted onto the elastic component in the direction of the z-axis, all the beams distort. When a moment acts on the elastic component about the z-axis, all the beams distort. When a moment acts on the elastic component about the x-axis or y-axis, the two beams disposed perpendicular to the moment-acting axis distort. The distortion of the beams is detected by the distortion detecting means such as strain gauges. Thus, the contact force and moment when grasping an object with the fingertips of a human being can be detected.